This invention relates to transmission controls for a power transmission.
Power transmissions of the automatic shifting type have a plurality of selectively establishable gear ratios between the engine and ground engaging mechanism for the vehicle. The ratios are generally established by hydraulically operated friction devices such as clutches and brakes. The engagement and disengagement of these friction devices are controlled by valve mechanisms which direct hydraulic fluid to and from the operating piston of the devices.
Some transmission control systems have incorporated electrohydraulic control systems with xe2x80x9cclutch to clutchxe2x80x9d shift technology. The control systems have utilized two strategies, open loop control and closed loop control. During open loop control, the on-coming friction torque transmitting mechanism (clutch or brake) is filled with fluid and the pressure is ramped up to the inertial pressure required during the shift. The release timing of the pressure in the off-going friction torque transmitting mechanism is based on an estimation of the oncoming torque transmitting mechanism fill time. The fill time of the oncoming torque transmitting mechanism varies due to many design and assembly factors such that the release of the off-going torque transmitting mechanism can be early, causing a flare, or late, causing a tie-up. Some control algorithms have been developed to detect the oncoming clutch fill using an input or output speed signal. However, these have not proved reliable for practical use.
During closed loop control, the off-going torque transmitting mechanism capacity is reduced to its critical point by generating a predetermined slip speed in the off-going torque transmitting mechanism. The oncoming torque transmitting mechanism is filled and ramped up to the inertial pressure. As the oncoming torque transmitting mechanism gains capacity, the input speed will drop. As the input speed drop is detected by the microprocessor, the off-going torque transmitting mechanism capacity is reduced to zero. In the closed loop control, there is a controlled engine flare at the beginning of the interchange causing an output torque dip. Also since the off-going torque transmitting mechanism is not released until the input speed drop is detected, a tie-up is present during the ratio interchange.
The present invention provides a transmission control system for a six speed automatic transmission. The control system provides clutch-to-clutch shift control with new features to improve robustness and failure mode.
The system minimizes the number of components while still ensuring the transmission is protected against any combination of actuator interruption.
The control system includes four pressure control valves, two low pressure ratios and two high pressure ratios. During an upshift transition, a switch valve directs an increasing pressure from a high ratio control valve to the on-coming friction device and a decreasing pressure from a low ratio control valve to the off-going clutch. A downshift valve is disposed in the circuit to direct the oncoming pressure to a bias port on the low ratio control valve to discontinue operation of the low ratio control valve and exhaust the of-going friction device. When the shift transition is complete, the switch valve directs fixed pressure to the engaged friction devices.
During a downshift, the downshift valve prevents the bias from the high ratio control valve, controlling the off-going friction device, from affecting the pressure output of the low ratio control valve which is controlling the on-coming friction device. An exhaust switching valve is provided to cooperate with the switch valve to insure that the proper friction device is pressurized and that the proper friction device is exhausted. During an up shift, the oncoming device (high ratio friction device) is maintained with a system pressure, and the off-going device (low ratio friction device) is supplied with a low (approx. 2 psi) fluid pressure. During a down shift, the opposite pressure schedule is required. The exhaust switching valve ensures that this will occur.